CLEAVAGE STRUCTURE. 



183 



Again, it has been compared to crystalline cleavage, on a huge scale. 

 It has been supposed that electricity traversing the earth in certain di- 

 rections, while certain rocks were in a semi-fluid or plastic state through 

 heat, arranged the particles of such rocks in a definite way, giving rise 

 to easy splitting in definite directions. In support of this view it was 

 urged that cleaved slates are most common in metamorphic regions ; 

 and metamorphism, as we shall see hereafter (p. 221 et seq.), indicates 

 the previous plastic state of rocks, which is a necessary condition of the 

 rearrangement of the particles by electricity. The great objections to 

 this theory are — 1. That the cleavage is not like crystalline cleavage, 

 between ultimate molecules, and therefore perfectly smooth, but be- 

 tween discrete and quite visible granules ; and, 2. That although the 

 phenomenon is indeed most common in metamorphic rocks, yet meta- 

 morphism is by no means a necessary condition ; on the contrary, when 

 the real necessary conditions are present, the less the metamorphism 

 the more perfect the cleavage. 



It is evident, therefore, that slaty cleavage is not due to any of the 

 causes spoken of above. It is not flag-stone cleavage, nor crystalline 

 cleavage, and of course can not be organic cleavage. 



Sharpe's Mechanical Theory. — The first decided step in the right 

 direction was made by Sharpe. According to him, slaty cleavage is al- 

 ways elite to powerful pressure at right angles to the planes of cleavage, 

 by which the pressed mass has been compressed in the direction of press- 

 ure and extended in the direction of the dip of the cleavage-planes. 

 This theory may be now regarded as completely established, by the 

 labors of Sharpe, Sorby, Haughton, Tyndall, and others. We will give 

 a few of the most important observations which establish its truth. 



(a.) Distorted Shells. — Many cleaved slates are full of fossils. In 

 such cases the fossils are always crushed and distorted as if by powerful 

 pressure, their diame- 

 ter being shortened 



at right angles to the 

 cleavage, and greatly 

 increased in the di- 

 rection of the cleav- 

 age-planes. The fol- 

 lowing figures (Fig. 

 160) are examples of 

 distortion by press- 

 ure. In Fig. 160, ZZ 

 gives the direction of FlG< 160 - DistOTted Fossils ( after snarpe). 



the planes of cleavage ; Figs. 1, 2, 3, 4, represent one species ; 5, 6, 7, 8, 

 another. In Fig. 161 still another species is represented in the natural 

 and distorted forms. 



